Is it a bird? Is it a plane? No! But it may not be a satellite either. There are a lot of things that move around above our heads, some are in our atmosphere, and some are out in space. So here’s how to find satellites, and how you can recognise one.

Motion

It’s not possible for something to stay stationary in space. It must move. If it doesn’t, it will fall straight back to Earth like a stone.

Contrary to popular belief, there is gravity in space, just like there is on Earth. Yes, the gravitational force between Earth and an object will be weaker further away from Earth than when the object is on the ground, but not enough to allow it to “float”.

Satellites are in orbit around Earth, which means they’re falling. This goes for every gravitationally-bound system: the Moon is falling around Earth, and the planets are falling around the Sun.

Imagine you have a cannon and you fire it. The ball will shoot out and land somewhere in the distance. Fire it with a bit more force, and it will land further out. Fire it with enough force and as it falls to the ground, it actually falls around Earth’s curve. The cannonball is now in orbit.

Newton’s Cannon

So basically, if you see a “satellite” and it’s not moving, then it’s not a satellite.

Also, a satellite moves in an elliptical orbit around Earth. When we observe them, they can curve gently as they move from horizon to horizon, but they’ll effectively be a straight line. Satellites never change direction suddenly.

Speed

A ‘standard’ orbit for a functioning satellite will have an altitude of ~300-400km, although this can vary greatly depending on the satellite. The orbital period at this altitude is just over an hour and a half (the International Space Station orbits Earth every ~93 minutes, and as of July 21st has an altitude of 403km). This speed only varies slightly, and can’t be seen with the naked eye.

So what does this mean for us when we see a satellite? Firstly, you’ll never see a satellite slow down or speed up. Secondly, the ISS travels from one horizon to the other in about 4 or 5 minutes. If you see something that crosses the entire sky in significantly less time, it means that it must be much closer to Earth. But satellites can’t orbit Earth much less than 300km, as atmospheric drag pulls them out of orbit and back to Earth.

However, the further away a satellite is from Earth, the longer it takes to orbit the planet, so you can expect it to cross the sky in a much shorter time. But because it’s further away it will generally appear dimmer, which brings us to…

Brightness

Satellites can vary in brightness, depending on size and how close they are to Earth. There are a couple of big satellites up there, the biggest being the International Space Station, but the vast majority of satellites can be approximated to being in or around the same size. Ish.

With this assumption, we can assume that if we see a dim satellite, it’s far away (or less likely, very small). If we see a bright one, it’s close (or big, but again, not likely).

“These are small, those are far away…”

Satellites aren’t very big. Some can be the size of a beer can (which you can’t see), with some being a bit bigger, maybe the size of a car. Some of these satellites are operational and actually do something, while others are space junk, and are bits of rocket casings left over from launches.

But whatever the satellite is, most satellites I have seen have been relatively dim, with the brighter ones being as bright as a normal, unassuming star. If you weren’t actually looking for the satellite, you probably wouldn’t notice it.

But as I mentioned, the ISS is a big one (it can fit in Croke Park) and it’s also quite close so it appears very bright bright. Some satellites – such as the Iridium satellites – have very reflective solar panels, and as they rotate, they can reflect a beam of light from space right down to an observer on the ground, making them look extremely bright.

Colour

This is probably the easiest test to figure out if your object is a satellite.

Satellites appear white. If it doesn’t appear white, then it’s not a satellite. This is because satellites reflect sunlight. While the Sun looks yellow (thanks to some awesome quantum physics, but that’s another story for another day), it’s a ‘broadband’ emitter, meaning it shines with all colours. When these colours blend together they appear white, much like the reverse of a beam of light going into a prism, if you remember the cover of Pink Floyd’s Dark Side of the Moon.

Ambiguous Descriptions

Sometimes when describing things we see in the sky, we don’t realise what we’re saying doesn’t really have any meaning. The main two culprits here are size and distance.

For example, you could say something looks “big” in the sky, but what does that mean? The Sun looks big, but so does a football I hold up in the air. When I look at Jupiter, it looks smaller than the Moon. But does that mean that the Moon is bigger than the biggest planet in the Solar System? This is another case of Fr Ted’s cows being small and far away.

Another one to do with size is that you can’t use normal measurements in the sky. If I said “the Moon looks a centimetre across” it would mean nothing. How far away is that centimetre from my eye? A euro coin held right up to my eye looks bigger than one at the bottom of the garden.

Distance is another thing that catches us out. For relatively small-appearing objects high up in the sky, we can’t tell how far away they are, because the distance between our eyes is so small compared to the distance between our eyes and the objects. Someone once told me that they saw a fireball “outside my window”, but fireballs burn out ~70km above the ground (fireballs are bright, so distance can be deceptive, and you’d very much know all about a fireball outside your window). This is the same reason why ancient astronomers couldn’t tell that the Moon was much closer to us than the Sun. There’s just no way to tell.

So even if something looks “big” or “close”, just be aware they very well might not be either of those things.

Summary

So that’s pretty much it. We now have a checklist that we can use to determine if what we’ve seen is a satellite.

For an object to be considered a satellite, it must:

  1. Move, and do so at a consistent and expected speed (not too fast)
  2. Be an expected brightness, roughly relating to its distance and therefore speed
  3. Be white

If your object fits all these, the congratulations! You’ve probably spotted a satellite!

The easiest and best way to start spotting satellites, and to get used to recognising them, is to simply know when they’re going to appear overhead. You can do this by visiting www.heavens-above.com.

When you visit, select your location on the map underneath ‘Configuration’. Next, select ‘Daily predictions for satellites brighter than magnitude 3.5, 4.0, 4.5’. The magnitude you choose is up to you, but to start off with I recommend selecting 3.5, then looking through the list to find the brightest ones (the lower the magnitude or brightness number, the brighter the satellite appears).

You will then be presented with a few columns. The first is the satellite list, the second is the starting point of the satellite with a rough direction where to look, the next is the highest point (0 degrees is the horizon, 90 degrees is straight up), and finally the ending point with a direction.

Common Misidentifications

There are a number of objects in the sky that are often misidentified as a satellite.

1. Venus and other bright planets

These look unusual, so are often mistaken for satellites. But, as we now know, satellites always move, whereas planets will appear stationary.

2. Fireballs and meteors

Sometimes debris from space can enter Earth’s atmosphere and burn up. However, these will streak across the sky in a matter of a few seconds, which is too fast to be a satellite in orbit. Also, meteors and fireballs will have a tail behind them, something satellites don’t have.

3. Asteroids and NEOs

There are objects that fit the checklist for satellites, but they are extremely rare. These are asteroids which pass very close to Earth, and are known as Near Earth Objects, or Potentially Hazardous Asteroids. Indeed, it could take hours for one of these to cross the sky, and would almost certainly be invisible to the naked eye.

4. Chinese lanterns

These are popular hot air balloons roughly the size of a beachball that can be bought all over the place. These can get caught in fast moving wind, even though on the ground the air is still. They can appear to move fast or slow, but we can’t really tell their distance. This wind can also change direction suddenly, making the lantern move about. The flame inside makes it look orange. These are definitely not satellites. And even though they look ball-y and fire-y, they’re not fireballs.

5. Aircraft

Aircraft often don’t change direction and can move at a similar speed to satellites, so they can be misidentified as such. However, look out closely for lights. If you see any flashing, it’s probably not a satellite (satellites don’t need flashing lights normally). Also look out for colour, as they should have red and/or blue lights (although if it’s far away these colours can fade and appear whiter).

The real trick here is to watch its speed. As it moves further away from you, it will look like it’s slowing down very, very gradually. That said, the ISS does the same thing, as it’s relatively close to Earth.

Aircraft are possibly the easiest to confuse with satellites, but with practice and a trained eye (plus checking Heavens Above as I mentioned) you should be able to get used to what’s an aircraft and what’s not.

6. “UFOs”

If you see something that still doesn’t fit the checklist or the explanations of other objects, it doesn’t mean that aliens are on their way, no matter how much you want to it to be. Sure, they could be (another post for another day), but what you see can be explained by a multitude of things not on this list. Refer back to the checklist: if it doesn’t match those, it’s based in our own atmosphere, and (probably) man-made or natural.